Single stream phase tracking during channel estimation in a very high throughput wireless MIMO communication system

ABSTRACT

In a multiple-input, multiple-output (MIMO) system, a wireless node&#39;s receive chain demodulation function is enhanced to include phase tracking. VHT Long Training Fields (LTFs) embedded in a frame preamble are used for phase tracking. Single stream pilot tones are added during transmission of VHT-LTFs. A receiver estimates the channel using the pilot tones in a first set of LTFs. A second set of LTFs are used to estimate the phase of the pilot tones using the estimated channel. The phase estimation is continuously applied to other received data tones throughout the VHT-LTFs of data symbols. Phase errors due to PLL mismatches and phase noise are reduced at reception, leading to better signal to noise ratio for different levels of drift and frequency offset. Further, MIMO channel estimation is more accurate, improving the overall wireless network when the accurate MIMO channel estimation data participates in calibration and handshake between wireless nodes.

RELATED APPLICATION

The present application is a continuation of U.S. patent applicationSer. No. 13/947,653 filed Jul. 22, 2013 and granted as U.S. Pat. No.8,884,818 to issue Nov. 11, 2014, which is a continuation of U.S. patentapplication Ser. No. 12/869,521 filed Aug. 26, 2010 and granted as U.S.Pat. No. 8,494,575 on Jul. 23, 2013, which applications are incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

This invention relates generally to communication systems. Inparticular, it advances better communication of information throughwireless communication systems by phase tracking using pilot tonesembedded in the preamble of transferred frames.

BACKGROUND

Advances in internet usage are leading to bandwidth demand increase inall sections of the network. One such advance has been in the wirelesslocal area network (LAN) area. Demand for wireless LAN has experiencedphenomenal growth. This demand has been driven by users connectingnotebook computers to networks at work or at mobile gathering places,among others. Growth has extended beyond the PC as well. Consumerapplications like music streaming, internet telephony, gaming andin-home video transmission are also fueling growth in bandwidth.

These demand increases on wireless LAN have spurred extensive standardsdevelopment in the technical area. Several wireless communicationsstandards such as the Institute of Electrical Engineers (IEEE) 802.11standard have emerged. IEEE 802.11 denotes a set of wireless local areanetwork (WLAN) air interface standards for short-range communicationsranging from tens of meters to a few hundred meters. One such WLANstandard is 802.11b. This standard specifies raw data rates up to 11Mbps using modulation techniques of Complementary Code Key (CCK) and/orDirect-Sequence Spread Spectrum (DSSS). The 802.11a standard, definedcontemporaneously with 802.11b, uses a more efficient method oftransmission called Orthogonal Frequency Division Multiplexing (OFDM).The 802.11a standard enabled data rates up to 54 mbps, but due toincompatible radio frequency band of 5 GHz, as compared to 2.4 GHz for802.11b, this standard was not widely deployed. In mid-year 2003, IEEEratified 802.11g, which applied OFDM modulation to the 2.4 GHz band.Much of WLAN client hardware supported both 802.11a and 802.11g

The follow on generation in the development of standards is 802.11n. The802.11n standard provides for a variety of optional modes that dictatedifferent maximum rates. The standard allows manufacturers to tunecapabilities to offer different price points and performance. The802.11n standard offers raw data rates up to 600 Mbps, while a 300 Mbpsspeed device could also be built consistent with 802.11n specification.

The 802.11n standard improves OFDM implementation using a higher maximumcode rate and higher bandwidth. It improves the raw rate from 54 Mbps to65 Mbps. Further, one of the widely known components of the standard isa Multiple Input Multiple Output or MIMO. MIMO exploits a radiophenomenon called multi-path. Multi-path involves transmittedinformation bouncing off doors, walls and other objects. Thisinformation reaches the receiving antenna multiple times throughdifferent paths and at slightly different times.

Multi-path degrades wireless performance if it is not controlled. MIMOtechnology, adopted in 802.11n standard, usefully deploys multi-paththrough space division multiplexing (SOMA). The WLAN transmitter devicesplits the data stream into multiple parts, called spatial streams. Eachspatial stream is transmitted through separate antennas to correspondingantennas on the receiver. The 802.11n supports up to 4 spatial streams.While doubling or quadrupling the spatial stream leads to increase inraw data rates, the cost and power also tend to increase due toincreased processing required per antenna pair. A MIMO system ischaracterized by the number of transmitter antennas by the number ofreceiver antennas. A 4×4 MIMO, for example, has four antennas on thetransmitter and 4 antennas on the receiver.

MIMO performance can be improved by beam-forming and diversity.Beam-forming directs the radio signal on to the target antenna. Thisimproves range and performance by limiting interference. Diversityexploits multiple antennas by combining the outputs of or selecting thebest subset of a larger number of antennas than required to receive acertain number of spatial streams. Excess antennas may be used to saycombine multiple received streams to one stream, operating over longerrange. Similar trade offs may be made for increasing raw data rates,with a fixed range.

The 802.11n standard, in summary, advances wireless LAN (WLAN) throughbetter OFDM characteristics, space division multiplexing through MIMO,diversity, power saving methods, doubling of channel from 20 MHz to 40MHz, MAC level aggregation of overhead, and reduced inter frame space.

In the follow on standards, referred to as 802.11 for Very HighThroughput (VHT) at 5 GHz band, the RF bandwidths targeted are up to 160MHz and data rates are up to 6.933 Gbps. More efficient signalprocessing schemes are being deployed to reduce noise and improve thesignal to noise ratio. Traditionally, pilot tones in the data symbolshave been used to perform phase tracking during data symbols, but for802.11n and above generations, this is compute costly and not feasibleduring MIMO channel estimation. The pilot tones as defined in 802.11nfor the Long Training Fields (LTFs) vary from stream to stream andtherefore cannot be used for accurate phase tracking.

For a 5 GHz carrier frequency, a 2 parts per million drift amounts to afrequency drift of 100 KHz. This frequency drift, with 4 symbols,amounts to a phase rotation of 5 degrees. For eight symbols, thisdoubles to 10 degrees. In OFDM, since the signal is carried in thephase, a phase drift leads to lower signal to noise ratio. Thisphenomenon makes the wireless network degrade on both performance andthroughput.

SUMMARY

Embodiments of the present invention pertain to phase tracking usingpilot tones in the preamble of the frame for a MIMO wirelesscommunication system.

In one embodiment, similar to pilot tones in the data symbols, pilottones in VHT-LTFs can be defined for phase tracking. Unlike the datatones, the MIMO training mapping cover sequence matrix (commonlyreferred to as a P matrix) is not applied to the pilot tones duringchannel estimation based on pilot tones. Instead, single stream pilotsare mapped to all space times stream (STS). In this embodiment, thepilot tones in the first VHT-LTF are used for initial one dimensionalchannel estimation. The pilot tones in the other remaining VHT-LTFs areused to estimate phase rotation based on pilot tones and the initial onedimensional channel estimation. The derived and consolidated informationis used for MIMO channel estimation for data tones.

In one embodiment of the present invention, a method and apparatus isdisclosed to use training fields in the header of a transmitted frame toestimate, on reception, the channel and the phase errors. Thisinformation is then applied to data tones to improve throughput andperformance.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are incorporated in and form a part of thisspecification. The drawings illustrate embodiments. Together with thedescription, the drawings serve to explain the principles of theembodiments.

FIG. 1 is a block diagram showing a typical Wireless LAN network in ahome or small business application.

FIG. 2 is a block diagram illustrating a wireless transmission andreception node and its components communicating through M transmit and Nreceive antennas.

FIG. 3 is an exemplary frame structure for a Physical Layer ConvergenceProtocol (PLCP) frame used in wireless communications.

FIG. 4 is a block diagram of a higher level node to node wirelesscommunication using channel estimation matrix for characterizingreception at each node based on transmitted information from the othernode.

FIG. 5 is a hardware block diagram of the phase tracking and correctionblocks coupled to the Fast Fourier Transform (FFT) component atreception, according to embodiments of the present invention.

FIG. 6 is an illustration of meshed pilot tones and data tones in OFDMsymbols, according to embodiments of the present invention.

FIG. 7 is a timeline diagram of signal processing in various hardwareblocks involved in extracting phase error information from pilot tonesand using the information n to correct phase rotation in data tones,according to embodiments of the present invention.

FIG. 8 is a flow chart representation of phase tracking using pilottones in the VHT-LTFs of the preamble, estimation of phase errors andits use in correcting the data tones prior to channel estimation,according to embodiments of the present invention.

DETAILED DESCRIPTION

Some portions of the detailed descriptions which follow are presented interms of procedures, logic blocks, processing and other symbolicrepresentations of operations on data bits within a computer memory.These descriptions and representations are the means used by thoseskilled in the data processing arts to most effectively convey thesubstance of their work to others skilled in the art. In the presentapplication, a procedure, logic block, process, or the like, isconceived to be a self-consistent sequence of steps or instructionsleading to a desired result. The steps are those requiring physicalmanipulations of physical quantities. Usually, although not necessarily,these quantities take the form of electrical or magnetic signals capableof being stored, transferred, combined, compared, and otherwisemanipulated in a computer system.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise as apparent from the followingdiscussions, it is appreciated that throughout the present application,discussions utilizing the terms such as “accessing,” “receiving,”“sending,” “using,” “selecting,” “determining,” “normalizing,”“multiplying,” “averaging,” “monitoring,” “comparing,” “applying,”“updating,” “measuring,” “deriving” or the like, refer to the actionsand processes of a computer system, or similar electronic computingdevice, that manipulates and transforms data represented as physical(electronic) quantities within the computer system's registers andmemories into other data similarly represented as physical quantitieswithin the computer system memories or registers or other suchinformation storage, transmission or display devices.

Embodiments described herein may be discussed in the general context ofcomputer-executable instructions residing on some form ofcomputer-usable medium, such as program modules, executed by one or morecomputers or other devices. Generally, program modules include routines,programs, objects, components, data structures, etc., that performparticular tasks or implement particular abstract data types. Thefunctionality of the program modules may be combined or distributed asdesired in various embodiments.

By way of example, and not limitation, computer-usable media maycomprise computer storage media and communication media. Computerstorage media includes volatile and nonvolatile, removable andnon-removable media implemented in any method or technology for storageof information such as computer-readable instructions, data structures,program modules or other data. Computer storage media includes, but isnot limited to, random access memory (RAM), read only memory (ROM),electrically erasable programmable ROM (EEPROM), flash memory or othermemory technology, compact disk ROM (CD-ROM), digital versatile disks(DVDs) or other optical storage, magnetic cassettes, magnetic tape,magnetic disk storage or other magnetic storage devices, or any othermedium that can be used to store the desired information.

Communication media can embody computer-readable instructions, datastructures, program modules or other data in a modulated data signalsuch as a carrier wave or other transport mechanism and includes anyinformation delivery media. The term “modulated data signal” means asignal that has one or more of its characteristics set or changed insuch a manner as to encode information in the signal. By way of example,and not limitation, communication media includes wired media such as awired network or direct-wired connection, and wireless media such asacoustic, radio frequency (RF), infrared and other wireless media.Combinations of any of the above should also be included within thescope of computer-readable media.

FIG. 1 100 is a block diagram of a typical wireless LAN network 105deployed at home or business. Several users are represented by stations130 among others. Stations are capable of receiving and transmittingdata from and to a base station 120. A wireless Access Point (AP) is oneembodiment of the base station. The base station 120 communicates with arouter 115 through a wire or wirelessly. The router 115 has networkconnectivity information on the network and receives and forwardspackets based on the source and destination addresses. A router has aplurality of ports for connections and a single uplink port to connectto the rest of the internet through a cable modem 110, generally througha wire 160. A cable modem connects to the world wide internet through aCable Modem Termination System (CMTS) located in a central office of theservice provider. Primarily, this invention deals with the wirelesscommunication 140 between a station 130 and base station 120. The new802.11 VHT standard proposes to transport data raw rates up to 6.933Gbps wirelessly and reliably over the air.

FIG. 2 is a block diagram of a wireless transmission and reception nodecomplex 250. A “to be transmitted” stream S is prepared based on payloaddata and it is encoded with a preamble and other information beforebeing fed into an encoder and modulator block 205. The node complexconsists of M antennas 220 in the transmit direction and N antennas 260on reception to form a M by N MIMO System. The node complex, whileoperating in the MIMO mode, may use, in one embodiment, spatial divisionmultiplexing (SOMA) to communicate with several receivers. SDMA enablesmultiple streams transmitted to different receivers at the same time toshare the same frequency spectrum. Within any stream, there are datapackets that contain both payload data and a preamble.

Simultaneous multiple stream transmission leads to higher bandwidth. Toachieve simultaneity, each data stream is spatially pre-coded and thentransmitted through a different transmit antenna. This spatialpre-coding and processing is done by block 210. This results in asequence of code symbols which are mapped to a signal group to produce asequence of modulation symbols.

The MIMO System may support a number of modulation schemes, includingOrthogonal Frequency Division Multiplexing. OFDM is a spread spectrumtechnique. It distributes data over a number of sub-carriers spacedapart at precise frequencies. The spacing is orthogonal and enables areceiver to recover data. This modulation technique may be employedusing any wireless standard including 802.11ac VHT. The OFDM modulator205 splits the modulation symbols into a number of parallel streams. Aninverse FFT is performed on each set of sub-carrier to produce timedomain OFDM symbols. The OFDM symbols are distributed in the payloads ofmultiple data packet. A preamble is carried along with the payload ineach data packet. The preamble comprises several symbols which are splitinto parallel streams similar to data. The preamble is appended to thedata payload prior to spatial processing. Different spatial streams aretransmitted through a plurality of antennas using RF transceivers 225.

The transmitted information is received on a plurality of antennas 260.This is fed into transceivers 206 to recover the information modulatedon RF carriers. The recovered information is provided to receive spatialprocessor 270. Data carried on any spatial streams are recovered. Apreamble processor uses the preamble to provide synchronizationinformation to OFDM demodulator and other downstream processing. TheOFDM demodulator 275 converts the stream from time domain to frequencydomain using Fast Fourier Transform (FFT). The frequency domain includesa stream per sub-carrier. The channel estimator 285 receives the streamand estimates the channel response. As part of the preamble, there arepilot tones which are phase shifted due to transmission through awireless channel. This is due to relative frequency residual offsetsbetween the PLLs at reception and transmission. The shift is generally alinear shift. Another phase shift occurs due to phase noise.

FIG. 3 represents a bidirectional node to node communication betweenstation A 300 and station B 350. The wireless channel between A and B ismathematically modeled by channel response matrix H_(AB), while the samein the other direction is modeled by matrix H_(BA). Through properhandshake and possible calibration, both stations compute correctionmatrix K_(A) and K_(B) to effectuate a reliable and high throughputwireless transmission.

As part of the demodulation, the pilot tones in the preamble aresubjected to special processing. FIG. 4 is an exemplary representationof a possible Physical Layer Convergence Protocol (PLCP) frame 400. Theframe consists of payload data packed as OFDM symbols as well aspreamble information. Part of the preamble information are the trainingsequences classified as “L” type for legacy and as “VHT” type for thenewly defined training sequences peculiar to new developing standards.One such training field is VHT-LTF (very-high throughputlong-training-field) 410. In a M by N MIMO system, the preamble willhave N VHT-LTFs. These symbols, like data symbols, include a mix ofknown training sequences at the position of pilot tones (predefineddata) and data tones. As described earlier, the OFDM transmit processorpre-pends the preamble in front of the packet data as part of formationof the “to be modulated” symbols.

In the wireless standards up to 802.11n, the pilot tones in the LTFs arepredefined for multiple space time but changing between the LTF streams(STS) which change across LTFs for 1, 2 and so on to L where L is thenumber of STS. Such a variation over time and spatial domain removes thepossibility of using the pilot tones in the LTFs for phase estimationand correction. As one embodiment of the invention, it is proposed thatthe pilot tones embedded in the VHT-LTFs be the same for space timestreams. As an exemplary embodiment, the P matrix (MIMO training coversequence) is replaced by the R matrix (receive signal matrix) whereinall rows of R matrix are identical to 1st row of P matrix. To avoidunintentional transmit beam forming, a per-stream cyclic shift delay(CSD) is still applied to all streams after the R mapping of the pilottones of the VHT-LTFs before applying per stream cyclic shift delay.This constancy is exploited by the invention to predict a onedimensional channel estimation from the first VHT-LTF. Other VHT-LTFsare used to perform phase estimation and the derived information isimmediately applied to correct phase of received LTFs at data tones. Inthe end, all VHT-LTFs are phase corrected. The phase error informationis fully consolidated to get and is applied to the data tones in thedata symbols for correction. This information is also used as one set ofinformation for channel estimation and determination of matrixH_(AB)/H_(BA) at the data tones.

In FIG. 2, on the receive side, this is illustrated by an added blockbetween the OFDM demodulator 275 and receive spatial processor 270,referred to herein as the phase tracker block 280. After receivinginformation from receive transceivers 265, via the receive spatialprocessor 270, the phase tracker block 280 along with demodulator 275,perform channel estimation 285 which is fed into downstream receiveprocessing 295.

The hardware components of the phase tracker block 500 are shown in FIG.5. The received time domain stream 550 is converted into the frequencydomain through a Fast Fourier Transform processor 502. The postprocessed pilot tone information is fed to a phase tracker 501. Thephase tracker is disabled during a first VHT-LTF and is enabled untilreception of the last VHT-LTF. Performing a channel estimation withoutusing the P matrix (training cover sequence matrix), phase correctioninformation is generated and consolidated using the pilot tones in theVHT-LTFs.

The estimated phases conveyed on 552 are multiplied 505 by data tonestream 552 to be fed into the channel estimation block 504. Data channelestimation information is generated on 555 to be conveyed to downstreamreceive processing. Due to added phase correction on data symbols, thechannel estimation is more accurate and less error prone. Suchestimation information when used at this node and a plurality of othernodes through handshaking and calibration, improves the overall wirelessnetwork performance.

FIG. 6 illustrates intermingling of pilot tones 601 and data tones 602in OFDM symbols. The phase estimation information is applied immediatelyto all data tones for correction.

It is assumed that the OFDM system is L dimensional and involves LongTraining Fields 1 through L. During the first VHT-LTF, the pilot tonesembedded are used to estimate the pilot communication channel in a onedimensional (single stream) way. Mathematically, after Fast FourierTransform, the receive signal in frequency domain is modeled as:

$\begin{matrix}{{r_{n,l}(k)} = {{\exp\left( {j\;\theta_{l}} \right)} \cdot {s(k)} \cdot {\sum\limits_{m = 1}^{M}\;{{h_{n,m}(k)}p_{m,l}}}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

In equation 1, k is the index of tone in the particular OFDM symbol, Iis the index of the OFDM symbol. The MIMO system is M by N dimensional,implying that the transmitter has M antennas and receiver has Nantennas, where the corresponding indices are m and n respectively. Theindex m ranges from 1 to M and the index n ranges from 1 to N. For the mtransmitter and n^(th) receiver, the channel response is representedmathematically as h_(n,m)(k) for tone k. s(k) is the channel trainingsequence at the k^(th) data tone while θ₁ is the phase rotation for theI^(th) symbol. p_(m,l) is the MIMO training cover sequence at m^(th)transmit antenna and I^(th) OFDM symbol. P, defined as P=[P_(m,l)] isthe entire MIMO training cover sequence.

Result wise, r_(n,l)(k) represents the received samples of n^(th)receive antenna at the k^(th) tone of the l^(th) OFDM symbol.Accordingly, R_(l)(k)=└r_(1,l)(k)r_(2,l)(k) Δ r_(N,l)(k)┘ is the entirereceive signal vector.

In one embodiment of the present invention, to track phase duringVHT-LTFs, MIMO training cover sequence P_(m,l) is not used at pilottones. Accordingly, deriving from Equation 1, receive pilot tones afterFFT can be modeled by:

$\begin{matrix}{{r_{n,l}(k)} = {{{\exp\left( {j\;\theta_{l}} \right)} \cdot {s(k)}}{\sum\limits_{m = 1}^{M}\;{h_{n,m}(k)}}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$where index k stands for the index of pilot tones. Due to missing P,only one dimensional channel is estimated at pilot tones.

$\begin{matrix}{{h_{n}(k)} = {\sum\limits_{m = 1}^{M}\;{h_{n,m}(k)}}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

Step 1: At first VHT-LTF, for each pilot tone k, one dimensional channelH(k) is estimated as:ĥ _(n)(k)=r _(n,l)(k)/s(k)  (Equation 4)

Step 2: For VHT-LTF 1 to I, phase rotation is estimated based on each ofthe pilot tones as:

$\begin{matrix}{{\hat{\theta}}_{l} = {\sum\limits_{k \in {\{{{pilot}\mspace{14mu}{tones}})}}\;{\sum\limits_{n = 1}^{N}\;{{r_{n,l}(k)} \cdot {{\hat{h}}_{n}(k)}}}}} & \left( {{Equation}\mspace{14mu} 5} \right)\end{matrix}$

Step 3: For data tones for VHT-LTF 1 to I, Equation 5 is used to correctthe phase of received (RX) data tones as:{tilde over (r)} _(n,l)(k)=r _(n,l)(k)·{circumflex over(θ)}_(l)  (Equation 6)Essentially, the k^(th) data tone vector is corrected using the phaseestimation from the pilot tones. These steps end with the I^(th)VHT-LTF.

In one embodiment of the present invention, for the balance of the datatones, MIMO channel estimation is calculated using the P matrix and thephase correction matrix for each 1 to M antenna to each 1 to N antennaaccording to the equation:Ĥ={tilde over (R)}·P ^(T)(P·P ^(T))⁻¹  (Equation 7)where

-   Ĥ(k)=[ĥ_(n,m)(k)]: MIMO channel estimation at k-data tone-   {tilde over (H)}(k)=[{tilde over (r)}_(n,m)(k)]: received signal    matrix with phase correction at k-^(th) data tone.

In one embodiment of the present invention, the channel estimationmatrix is a function of the receive vector for the k^(th) tone and thecorresponding phase correction applied with derived information from thepilot tones in the VHT-LTFs. With this correction and the use of channelestimation so derived, the receive signal to noise ratio is seen toincrease due to cancellation of both linear and non-linear phase errors.

FIG. 7 illustrates the timeline of signal processing involved in thephase tracking and correction based on the VHT-LTFs. In one embodimentof the present invention, during the first VHT-LTF 701, phase trackingblock 704 is involved in the one dimensional estimation of the channel,the phase correction is off and the MIMO channel estimation block 706 isin the buffering phase 707. During the second 702 to L−1 VHT-LTF 708,the phase tracking block estimates the phase and phase correction fordata tones is enabled. The channel estimation block 706 remains in thebuffering mode. During the L^(th) VHT-LTF 703, the phase estimationends, the phase correction 705 for data tones continues and the MIMOchannel estimation is enabled at the end of 709.

FIG. 8 captures the above steps in terms of a flow chart 800. In oneembodiment of the present invention, the receive stream derived from theradio frequency transceivers after reception from the antenna is fed toreceive spatial processors. After processing from the spatialprocessors, the stream enters the phase tracker and OFDM demodulatorblock where the preamble processing takes place 801. If decision block802 determines a first VHT-LTF, for each of its pilot tones, the onedimensional receive channel is estimated without use of P matrix as thecoverage sequence matrix for the pilot tones has identical values 804.If decision block 802 determines a non-first VHT-LTF, blocks 803 and 805perform estimation for pilot tones and phase correction for data tones,which is continued through decision block 808 until the last VHT-LTF.

For the data symbols, channel estimation is performed 809 till the lastdata symbol in the frame through decision block 807 after which thechannel estimation is disabled 806 and preamble processing begins forthe next frame through reentry into 801. The end product of the steps isa more accurate channel estimation matrix for the m by n dimension whichis used by downstream receive processing and for handshaking with othernodes.

In the foregoing specification, embodiments have been described withreference to numerous specific details that may vary from implementationto implementation. Thus, the sole and exclusive indicator of what is theinvention, and is intended by the applicant to be the invention, is theset of claims that issue from this application, in the specific form inwhich such claims issue, including any subsequent correction. Hence, nolimitation, element, property, feature, advantage, or attribute that isnot expressly recited in a claim should limit the scope of such claim inany way. Accordingly, the specification and drawings are to be regardedin an illustrative rather than a restrictive sense.

We claim:
 1. A wireless communication apparatus comprising: a processorconfigured to embed a set of identical information within each of aplurality of very-high throughput long-training-fields (VHT-LTFs)included in a preamble of a data frame; a modulator configured todistribute the plurality of VHT-LTFs and payload data of the data frameinto a plurality of spatial streams; and a plurality of transceiversconfigured to output the plurality of spatial streams for transmissionvia a plurality of antennas.
 2. The apparatus of claim 1, wherein theset of identical information comprises a single stream of identicalpilot tones.
 3. The apparatus of claim 1, wherein the modulator isconfigured to distribute the plurality of VHT-LTFs and payload datausing orthogonal frequency division multiplexing (OFDM).
 4. Theapparatus of claim 1, wherein the preamble of the data frame containsone VHT-LTF for each receive antenna of a device configured to receivethe plurality of spatial streams.
 5. The apparatus of claim 1, furthercomprising a spatial processor to apply a per-stream cyclic shift delayto each of the plurality of spatial streams.
 6. A method for wirelesscommunication, comprising: embedding a set of identical informationwithin each of a plurality of very-high throughput long-training-fields(VHT-LTFs) included in a preamble of a data frame; distributing theplurality of VHT-LTFs and payload data of the data frame into aplurality of spatial streams; and outputting the plurality of spatialstreams for transmission via a plurality of antennas.
 7. The method ofclaim 6, wherein the set of identical information comprises a singlestream of identical pilot tones.
 8. The method of claim 6, wherein theplurality of VHT-LTFs and payload data are distributed using orthogonalfrequency division multiplexing (OFDM).
 9. The method of claim 6,wherein the preamble of the data frame contains one VHT-LTF for eachreceive antenna of a device configured to receive the plurality ofspatial streams.
 10. The method of claim 6, further comprising: applyinga per-stream cyclic shift delay to each of the spatial streams prior totransmission.
 11. A non-transitory computer readable medium comprisinginstructions that, when executed by an apparatus, cause the apparatusto: embed a set of identical information within each of a plurality ofvery-high throughput long-training-fields (VHT-LTFs) included in apreamble of a data frame; distribute the plurality of VHT-LTFs andpayload data of the data frame into a plurality of spatial streams; andoutput the plurality of spatial streams for transmission via a pluralityof antennas.
 12. The non-transitory computer readable medium of claim11, wherein the set of identical information comprises a single streamof pilot tones.
 13. The non-transitory computer readable medium of claim11, wherein the plurality of VHT-LTFs and payload data are distributedusing orthogonal frequency division multiplexing (OFDM).
 14. Thenon-transitory computer readable medium of claim 11, wherein thepreamble of the data frame contains one VHT-LTF for each receive antennaof a device configured to receive the plurality of spatial streams. 15.The non-transitory computer readable medium of claim 11, whereinexecution of the instructions further causes the apparatus to apply aper-stream cyclic shift delay to each of the plurality of spatialstreams.